1. Field of the Invention
The invention relates to a method and an arrangement for decoding a predetermined code word.
2. Description of the Related Art
In the decoding of a code word which has a predetermined number of positions, the information-carrying positions are restored as completely as possible.
The decoding takes place at the end of the receiver which has received the code word via a disturbed channel. Signals are transmitted, in particular as Boolean values, preferably subdivided into +1 and −1, via the channel where they are subject to a disturbance, and are converted into analog values which can deviate to a greater or lesser extent from the predetermined Boolean values (±1) by a demodulator.
The general assumption is K positions of binary information (“information bits”) without redundancy uε{±1}K, which is mapped into a code word cε{±1}N by means of systematic block codes or unsystematic block codes by a channel coder. In this arrangement, the code word contains N−K bits (also “check bits”) which can be used as redundant information to the N information bits for restoring the information after transmission via the disturbed channel.
The systematic block code adds to the N information bits N−K check bits which are calculated from the information bits, the information bits themselves remaining unchanged whereas, in the unsystematic block code, the information bits themselves are changed, for example the information is in an operation performed from one to the next position. Here, too, check bits are provided for reconstructing the information hidden in the operations. In the text which follows, in particular, a technically significant variant of unsystematic block codes, the so-called terminated convolutional codes, is considered.
“Hard” decoding of a correlation of the received code word (with the positions occupied by analog values) i.e., correlating each position with the nearest Boolean value in each case, is a decisive disadvantage since valuable information is lost in this process.